Catalytic hydrogenation in supercritical CO2: Kinetic measurements in a gradientless internal-recycle reactor

Catalytic hydrogenation in supercritical carbon dioxide has been studied. Experimental results and theoretical calculations are presented, attempting to elucidate the effects of temperature, pressure, and CO2 concentration on the rate of reaction. An effort has been made to develop a procedure to test catalytic reactions in supercritical fluids in view of future industrial applications. Hydrogenation rates of the two double bonds of an unsaturated ketone on a commercial alumina-supported palladium catalyst were measured in a continuous gradientless internal-recycle reactor at different temperatures, pressures, and CO2-to-feed ratios. A Berty-type reactor has been suitably modified for use with supercritical solvents. The accurate control of the organic, carbon dioxide, and hydrogen feed flow rates and of the temperature and pressure inside the reactor provided reproducible values of the product stream compositions, which were measured on-line after separation of the gaseous components. In order to develop a kinetic model, vapor-liquid equilibrium calculations were carried out through a Peng-Robinson equation of state, tuned on binary high-pressure vapor-liquid equilibrium data. Compositions in the liquid phase inside the reactor were predicted, starting from the on-line analysis after depressurization. A simplified power law kinetic equation is shown to provide a good description of the experimental data.